In general, either direct dyes or oxidation dyes are used to color keratin fibers. With oxidation dyes, while intense colorations with good fastness characteristics can be achieved, the color is generally developed under the influence of oxidizing agents such as H2O2, for example, which can result in the damaging of the fibers in many cases. Moreover, many oxidation dye precursors or certain mixtures of oxidation dye precursors can have a sensitizing effect on people with sensitive skin. Direct dyes are applied under gentler conditions. Their disadvantage lies in the fact that the colorations often have only inadequate fastness characteristics.
A person skilled in the art will use direct dyes of different dye classes depending on the desired coloration result. The direct dyes known from the prior art belong, for example, to the class of nitro dyes, anthraquinone dyes, azo dyes, triarylmethane dyes, or methine dyes. All of these dye classes must satisfy a certain series of requirements in order to be used in the area of cosmetics. For instance, direct dyes should yield an intense coloring result and have fastness characteristics that are as good as possible. The coloration result obtained with direct dyes should be affected as little as possible by environmental influences that is, the dyes should have good wash fastness, light fastness, and rub fastness. Chemical influences to which the keratin fibers may be exposed after the dyeing process (such as permanent waves, for example) should also alter the coloration result as little as possible.
In order to also achieve lightening with the coloration at the same time, the direct dyes should, if possible, also be compatible with the oxidizing agents usually used in blonding processes (such as hydrogen peroxide and/or persulfates, for example).
For more pronounced lightening of dark hair, not only hydrogen peroxide alone, but a combination of hydrogen peroxide and persulfates (e.g., ammonium persulfate, potassium persulfate, and/or sodium persulfate) is used. So, if dark hair is to be dramatically lightened and simultaneously colored in a bright tone, it is advantageous to use a mixture of hydrogen peroxide, persulfates, and a direct dye. Although many intensively coloring direct dyes for dyeing hair are known to a person skilled in the art, he is familiar with only a very limited selection of dyes that can withstand the strong oxidative conditions such as those created through the mixing of the abovementioned oxidizing agents. What is more, the oxidation-stable dyes known from the prior art have serious drawbacks in terms of their fastness characteristics.
A need therefore continues to exist for dyes that are highly stable in the presence of strong oxidizing agents in order to enable the simultaneous dyeing and pronounced lightening of hair. Even under these extreme conditions of use, these dyes must not lose their positive fastness and coloring characteristics.
It has been found that bright and intense colorations can be produced using cationic direct dyes in particular. Cationic dyes are often characterized by an especially high affinity for keratin fibers, which can be attributed to the interactions of the positive charges of the dyes with negatively charged structural components of the keratin fibers. This is why it is often possible to achieve especially intense colorations using cationic dyes.
Two examples of monomeric cationic azo dyes that are sufficiently known from the prior art are Basic Orange 31 (alternative name: 2-[(4-aminophenyl)azo]-1,3-dimethyl-1H-imidazolium chloride, CAS no. 97404-02-9) and Basic Red 51 (alternative name: 2-[((4-dimethylamino)phenyl)azo]-1,3-dimethyl-1H-imidazolium chloride, CAS no. 77061-58-6).
Both dyes color keratin fibers with outstanding color intensity in the orange to red nuance range. Moreover, a need also continues to exist for direct blue dyes that are optimally compatible with these two dyes.